Cellular communication systems are currently being developed and improved for machine type communications (MTC). MTC is characterized by lower demands on data rates than for example mobile broadband, but generally has more stringent requirements regarding cost, service coverage, power. For example, a MTC device may be intended to operate for years on batteries, without charging or replacing the batteries.
Currently, the Third Generation Partnership Project (3GPP) is standardizing Enhanced Machine-Type Communications (eMTC) as well as Narrowband Internet of Things (NB-IoT) as part of LTE Release 13 for satisfying all the requirements put forward by MTC type applications, while maintaining backward compatibility with the current LTE radio access technology. See RP-150492 (3GPP TSG RAN Meeting #67, Shanghai, China, 9-12 Mar. 2015) regarding eMTC, and see RP-152284 (3GPP TSG RAN Meeting #70, Sitges, Spain, Dec. 7-10, 2015) regarding NB-IoT.
The eMTC features specified in RP-152024 and in R1-157926 (3GPP TSG RAN Meeting #70, Sitges, Spain, Dec. 7-10, 2015) include a low-complexity User Equipment (UE) category called UE category M1 (or Cat-M1 for short) and Coverage Enhancement (CE) techniques involving CE modes A and B that can be used together with UE category M1 or any other LTE UE category.
All eMTC features (both Cat-M1 and CE modes A and B) operate using a reduced maximum channel bandwidth compared to normal LTE. The maximum channel bandwidth in eMTC is 1.4 MHz, whereas it is up to 20 MHz in normal LTE. The eMTC UEs are still able to operate within the larger LTE system bandwidth without problem. The main difference compared to normal LTE UEs is that the eMTCs can only be scheduled with 6 Physical Resource Blocks (PRBs), 180 kHz at a time.
In CE modes A and B, the coverage of physical channels is enhanced through various coverage enhancement techniques, the most important being repetition or retransmission. In its simplest form, this means that the 1-ms subframe to be transmitted is repeated a number of times, e.g., just a few times if a small coverage enhancement is needed or hundreds or thousands of times if a large coverage enhancement is needed.
The objective of NB-IoT is to specify a radio access for cellular IoT, based to a great extent on a non-backward-compatible variant of E-UTRA, that addresses improved indoor coverage, support for massive number of low throughput devices, low delay sensitivity, ultra-low device cost, low device power consumption and (optimized) network architecture. Here, “E-UTRA” denotes “Evolved UMTS Terrestrial Radio Access” and “UMTS” denotes the “Universal Mobile Telecommunications Service.”
The NB-IoT carrier bandwidth is 200 KHz, and examples of operating bandwidth for LTE are 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz, etc. NB-IoT supports three different modes of operation. A first mode, “Stand-alone operation,” uses for example the spectrum currently being used by GSM Edge Radio Access Network (GERAN) systems as a replacement of one or more GSM carriers, where “GSM” denotes the “Global System for Mobile Communications.” In principle, NB-IoT operates on any carrier frequency which is neither within the carrier of another system and not within the guard band of another system's operating carrier. The other system can be another NB-IoT system or a system associated with another Radio Access Technology (RAT), such as a LTE system. A second mode, “Guard band operation,” uses the unused resource blocks within a LTE carrier's guard band. The term “guard band mode” may be used interchangeably with the term “guard bandwidth.” As an example, in case of LTE BW of 20 MHz (100 Resource Blocks or RBs), the guard band operation of NB-IoT can be placed anywhere outside the central 18 MHz but within 20 MHz LTE BW.
A third, mode contemplated for NB-IoT, referred to as “In-band operation,” uses resource blocks within a normal LTE carrier. The in-band operation mode may also be referred to as in-bandwidth operation. Broadly, the operation of one RAT within the BW of another RAT is referred to as in-band operation. As an example, in a LTE BW of 50 RBs (50 RBs), NB-IoT operation over one resource block (RB) within the 50 RBs is called in-band operation.
In NB-IoT, the downlink transmission is based on OFDM with 15 kHz subcarrier spacing for all the scenarios: standalone, guard-band, and in-band. For UL transmission, both multi-tone transmissions based on SC-FDMA, and single tone transmission is supported. This means that the physical waveforms for NB-IoT in the downlink and also partly in the uplink are similar to those seen in LTE.
In the downlink design, NB-IoT supports both master information broadcast and system information broadcast, of which broadcasts are carried by different physical channels. For in-band operation, it is possible for a NB-IoT UE to decode the NB Physical Broadcast Channel (NB-PBCH) without knowing the legacy Physical Radio Block (PRB) index. NB-IoT supports both downlink physical control channel (NB-PDCCH, or NB-M-PDCCH) and downlink physical shared channel (PDSCH). The operating mode of NB-IoT must be indicated to the UE, and currently 3GPP considers indication by means of NB Secondary Synchronization Signal (NB-SSS), NB Master Information Block (NB-MIB), or perhaps via other downlink signals.
The references signals used in NB-IoT are expected to follow the general design principles seen in LTE. For example, downlink synchronization signals for NB-IoT operation may include a NB Primary Synchronization Signal (NB-PSS) and a NB-SSS.
In half duplex (HD) or more specifically. HD Frequency Division Duplex (FDD), the uplink (UL) and downlink (DL) transmissions take place on different paired carrier frequencies but not simultaneously in time in the same cell. This means the UL and DL transmissions take place in different time resources. Examples of time resource are symbols, time slots, subframes, transmission time intervals (TTIs), interleaving times, etc. In other words. UL and DL (e.g., subframes) do not overlap in time. The number and location of subframes used for DL, UL or unused subframes can vary on the basis of a frame or a multiple of frames. For example, in one radio frame (say frame #1) subframes #9, #0, #4 and #5 are used for DL, and subframes #2 and #7 are used for UL transmission. But in another frame (say frame #2) subframes #0 and #5 are used for DL and subframes #2, #3, #7 and #8 are used for UL transmission.
One issue recognized herein is that a NB-IoT or eMTC device (henceforth a wireless communication device or “WCD”) operating in half duplex (HD) mode will lose frequency synchronization to the network node when configured with a high repetition factor for uplink transmissions. The repetition factor may be referred to as “bundle size.” Affected uplink transmissions include: (a) UL control channel transmission, e.g., PUCCH for eMTC and NB-PUCCH for NB-IoT: (b) UL data channel transmission, e.g., PUSCH for eMTC and NB-PUSCH for NB-IoT; (c) physical random access channel transmission, PRACH for eMTC and NB-PRACH. For eMTC, the repetition factor may be as large as 2048, meaning that the transmission is repeated in 2048 consecutive UL subframes (or more if there are invalid UL subframes). For NB-IoT devices, operating over a smaller bandwidth (one sixth of the eMTC bandwidth), even larger repetition factors are anticipated. During the time of the repeated transmissions, a WCD operating in HD cannot receive on the downlink, and, therefore, cannot stay tuned to the downlink carrier frequency.
Large repetition factors are typically used to facilitate maintained connectivity when the WCD is operating in an enhanced or extreme coverage scenario. For example, assume that a given value or lower range of received signal level defines “normal” coverage operation. Enhanced coverage operation may then be understood as operating with lower-than-normal received signal levels, e.g., Signal-to-Noise-and-Interference-Ratios (SINRs) down to −15 dB. There may be further levels of enhanced coverage, such as an extreme coverage scenario involving −15 dB>SINR>=−20 dB.
High repetition factors—large bundle sizes—provide for operation in such coverage scenarios, but it is recognized herein that large bundle sizes create other problems. For example, UL transmissions with larger repetition result in frequency drift at the transmitting HD WDC because: 1) the extended time the HD WCD is tuned away from the downlink carrier, and 2) heating of circuitry in the HD WCD from continuous operation of the power amplifiers (PAs) in the HD WCD. When operating in enhanced or extreme coverage, the HD WCD typically uses its maximum transmit (TX) power, making the PA by far the largest power consumer in the modem of the HD WCD. For example, the power consumed by the PA during such times may be at least an order of magnitude larger than the whole baseband circuitry of the HD WCD. As a further example, a commercial Cat4 LTE modem operating with full throughput on downlink and uplink and using maximum transmit power uses about 57% of its total power on the PA and only 7% and 9% on DL and UL baseband processing, respectively.
The temperature changes impact the voltage controlled crystal oscillator (VCXO) of the HD WCD. The VCXO frequency increases or decreases in dependence on the temperature and on the time derivative of the temperature. See FIG. 1, which plots example behavior.
Crystal oscillators that are temperature compensated, referred to as TCXOs, display less frequency deviation in response to temperature changes, but TCXOs have drawbacks when it comes to their frequency response to voltage control. Plus, TXCOs are more expensive than VCXOs. Hence, WCD vendors prefer VCXOs. This fact is particularly important for NB-IoT and eMTC devices, because low device cost is required for achieving a significant penetration of their intended markets.
To achieve acceptable performance when receiving low-power or low-level signals, a network base station, such as an eNB or eNodeB in LTE parlance, accumulates the repeated transmissions from a HD WCD directly before rate restoration, after rate restoration and/or after attempting to decode the message (decoder branch metrics of soft bits). The effect of frequency drift at the HD WCD over the course of its repeated uplink transmissions introduces a gradual change in the received messages at the base station, even though the radio channel as such may be stationary. The change may introduce destructiveness in the coherent combining of sequentially received messages, and will additionally contribute to inter-carrier interference (ICI) and inter-symbol interference (ISI). At the same time, due to the enhanced or extreme coverage scenario and the resulting low SNR operating point, it is challenging for the network node to estimate and compensate for the drifting WCD carrier frequency.
Moreover, the frequency drift in the HD WCD results in time drift, whereby the timing at which the Orthogonal Frequency Division Multiplex (OFDM) symbols, or single- or multi Lone symbols transmitted by it gradually changes. Those changes lead to inter-symbol interference (ISI), as well as to a gradual phase change of the received signal (linear phase). Both ISI and gradually changing linear phase introduce additional destructiveness in the coherent accumulation carried out by the receiving network node.